Method and apparatus for controlling equivalent isotropic radiated power

ABSTRACT

An access point of a wireless communication network forms beams by applying a weightset for a beamforming weights matrix to signal streams. The Equivalent Isotropic Radiated Power (EIRP) that is emitted from an array of antenna elements at the access point is controlled by calibrating the transmission phase and gain of a respective transmit chain for each antenna element, providing a polar radiation model for an antenna element of the array, and determining a weightset for the weighting matrix subject to a constraint that a maximum total EIRP for the beams in combination in any azimuth direction is maintained within a predetermined EIRP limit, based at least on a spatial separation of the antenna elements, the polar radiation model and the calibrated transmission phase and gain of each respective transmit chain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/041,321, filed on Jul. 20, 2018, issuing as U.S. Pat. No. 10,355,764on Jul. 16, 2019, which is a continuation of U.S. application Ser. No.15/726,127, filed on Oct. 5, 2017, now U.S. Pat. No. 10,033,450 issuedon Jul. 24, 2018, which is a continuation of U.S. application Ser. No.15/074,767, filed on Mar. 18, 2016, now U.S. Pat. No. 9,787,386 issuedon Oct. 10, 2017, which claims the benefit of UK Application No. GB1518778.4, filed Oct. 23, 2015, all of which are incorporated byreference in their entirety.

TECHNICAL FIELD

The present invention relates generally to an access point in a point tomultipoint wireless network and to methods of operating the accesspoint, and more specifically, but not exclusively, to a method ofcontrolling the Equivalent Isotropic Radiated Power (EIRP) emitted froman array of antenna elements at an access point of a wirelesscommunication network.

BACKGROUND

Modern wireless communication networks are typically placed under greatdemands to provide high data capacity within the constraints of theallocated signal frequency spectrum. In cellular wireless communicationnetworks, capacity may be increased by re-using frequencies betweencells, typically according to a predetermined frequency re-use pattern.A fixed wireless access system may comprise a base station, which may bereferred to as an access point, typically mounted on an antenna tower,and a number of subscriber modules installed at customer premises. Thearea of coverage of an access point may be divided into sectors, eachsector being used to transmit and receive with a respective cell.Capacity may be further increased within a cell by steering respectivebeams towards specific user equipments, which may be referred to assubscriber modules, to allow communication between the access point withimproved gain and/or reduced interference reception in comparison with abeam covering a sector. The access point may be equipped with an antennaarray and a beamformer for each sector, for forming beams forcommunication with each respective subscriber module. The beamformer maybe required to form beams of various beamwidths in different modes ofoperation.

It may be a requirement to maintain effective isotropic radiated power(EIRP) within a predetermined limit. This may be achieved by limitingthe power transmitted to each antenna element of the antenna array to alevel such that, if the worst case maximum array gain were applied bythe beamformer, the EIRP limit would not be exceeded. However, thisapproach has the disadvantage that the access point may be caused totransmit at a level significantly below the EIRP limit for beamformersettings that do not apply the maximum array gain. This may limit thesignal to noise ratio achievable on a link and so limit system datacapacity.

It is an object of the invention to mitigate the problems of the priorart.

SUMMARY

In accordance with a first aspect of the invention there is provided amethod of controlling the Equivalent Isotropic Radiated Power (EIRP)emitted from an array of antenna elements at an access point of awireless communication network, the access point being configured toform one or more beams by applying a weightset for a beamforming weightsmatrix to one or more signal streams in a first mode of operation, themethod comprising:

calibrating transmission phase and gain of a respective transmit chainfor each antenna element;

providing a polar radiation model for an antenna element of the array;and

determining a weightset for the weighting matrix subject to a constraintthat a maximum total EIRP for the one or more beams in combination inany azimuth direction is maintained within a predetermined EIRP limit,

wherein said determining is based at least on a spatial separation ofthe antenna elements, the polar radiation model and the calibratedtransmission phase and gain of each respective transmit chain.

This allows the Equivalent Isotropic Radiated Power (EIRP) emitted froman array of antenna elements to be reliably controlled for a variety ofbeam shapes, so that the access point may transmit closer to an EIRPlimit without risking exceeding the limit. It has been found that, inparticular, variations in the relative transmission phase of thetransmit chains may affect the array gain. Calibration of the respectivetransmit chains allows variations of phase and gain of each transmitchain with time and/or temperature to be accounted for when calculatingEIRP, allowing operation closer to an EIRP limit than would be the caseif the transmit chains were not calibrated.

In an embodiment of the invention, said determining comprises:

determining a first weightset for forming the one or more beams; and

amending the first weightset so that the maximum total EIRP for the oneor more beams in combination in any azimuth direction is maintainedwithin the predetermined EIRP limit.

This allows a first weightset to be determined, for example by abeamfoming process that may optimise a signal to noise ratio to a givensubscriber module, and the first weightset may then be amended, forexample by applying a gain factor to the weightset, to maintain the EIRPwithin the predetermined limit when transmitted. This allows theflexibility of generating narrow beams while maintaining operationwithin an EIRP limit.

In an embodiment of the invention, amending the first weightsetcomprises:

determining the maximum total EIRP for the one or more beams incombination in any azimuth direction;

comparing the determined maximum total EIRP for the one or more beamswith a predetermined EIRP limit;

in dependence on the maximum total EIRP of the one or more beamsexceeding the predetermined EIRP limit, amending the first weightset toreduce a gain factor for at least one of the one or more signal streamsto reduce the maximum EIRP of the first beam to be within the EIRPlimit.

This allows selective reduction of the EIRP of respective beams.

In an embodiment of the invention, said calibrating of the respectivetransmit chains is as a function of frequency, and wherein the gainfactor is a matrix of gain as a function of frequency.

This allows gain to be maintained at frequencies that do not exceed theEIRP limit, improving overall signal to noise ratio.

In an embodiment of the invention, said calibrating of the respectivetransmit chains comprises:

generating an Orthogonal Frequency Division Multiplexing (OFDM) testsymbol for each transmit chain;

combining signals from each transmit chain into a combined channel;

receiving a combined OFDM symbol in the combined channel, the combinedOFDM symbol comprising respective subcarriers transmitted by respectivetransmit chains; and

calibrating each transmit chain based on the received respectivesubcarriers in the combined OFDM symbol.

This allows calibration to be performed using a simple passive combiner.

In an embodiment of the invention, each respective OFDM test symbolcomprises a respective set of energised subcarriers.

This allows the test symbols to be received without interference betweenthe test symbols.

In an embodiment of the invention, a relationship between the OFDM testsymbols is characterised by a Hermitian matrix.

This allows orthogonal results to be derived for the transmissioncharacteristics of the transmit chains.

This allows a simple receiver architecture to be implemented using acombiner, and reduces test time by enabling the test symbols to bereceived simultaneously without interference between the test symbols.

In an embodiment of the invention, the method comprises providing anisolation between antenna elements of at least 30 dB.

This allows an accurate model of EIRP to be determined without modellinginteraction between antenna elements, so that the access point may beoperated nearer to a predetermined EIRP limit.

In an embodiment of the invention, the method comprises performing saidcalibrating of the transmission phase and gain of respective transmitchains periodically as part of a time frame sequence including timeframes for the transmission of payload data.

This allows variations of the gain and/or phase of the transmit chainswith time and/or temperature to be calibrated.

In an embodiment of the invention, the period between performance ofcalibration is less than or equal to 64 time division duplex frames.

This allows accurate calibration.

In an embodiment of the invention, the period is 8 to 32 time divisionduplex frames.

This has been found to offer a good trade off between calibrationaccuracy and throughput of payload data, which may be inhibited duringcalibration.

In an embodiment of the invention, the method comprises providing theantenna array, the respective transmit chains and a combiner network forthe combined channel as a unit within a single enclosure arranged toimpede changing of the relative special arrangement of the antennaelements by an operator, whereby to maintain a predeterminedconfiguration of the antenna array to enable accurate determination ofmaximum EIRP.

This allows an accurate determination of EIRP to be maintained afterdelivery to an operator.

In an embodiment of the invention, the method comprises connecting eachrespective transmit chain to the antenna array without use of coaxialcable connectors.

This allows accurate determination of EIRP to be maintained afterdelivery to an operator by preventing reconfiguration of the equipment.

In an embodiment of the invention, the method comprises:

switching from a first mode of operation to a second mode of operation;

in the second mode of operation, configuring one or more beams byapplying a second weightset for the beamforming weights matrix to theone or more signal streams,

wherein the second weightset is determined subject to a constraint thata maximum total EIRP for the one or more beams in combination in anyazimuth direction is maintained within the same predetermined EIRP limitas for the first mode.

This allows flexibility of operation in terms of beamwidth whilemaintaining transmitted power within an EIRP limit.

In an embodiment of the invention, the first mode is a sector mode inwhich signals from the respective transceiver chains are combined toform a beam sufficiently broad to provide coverage of a sector of acellular radio network; and

the second mode is a combining mode, in which signals from therespective transceiver chains are combined to form a beam which isnarrower in azimuth than that formed in the sector mode to provide abeam steered to an individual subscriber mode within the sector of acellular radio network.

This allows the flexibility of switching to a beamformed mode ofoperation, for example for communication to a given subscriber module,while maintaining transmitted power within an EIRP limit.

In an embodiment of the invention, the first mode is a sector mode inwhich signals from the respective transceiver chains are combined toform a beam sufficiently broad to provide coverage of a sector of acellular radio network; and

the second mode is a Multiple User Multiple Input Multiple Output(MU-MIMO) mode, in which signals from the respective transceiver chainsare combined to form at least two beams carrying different data torespective subscriber modules within the sector of a cellular radionetwork.

This allows the flexibility of switching to a MU-MIMO mode of operation,for example for communication of respective data streams to severalsubscriber modules, while maintaining total transmitted power for theMU-MIMO beams within an EIRP limit in all azimuth directions.

In an embodiment of the invention, each weightset comprises respectiveamplitude and phase values for respective signal streams for respectiveantenna elements for respective sub-carriers of an OFDM symbol.

This allows beamforming to take into frequency dependent effects.

In accordance with a second aspect of the invention, there is providedan access point for a wireless communication network, the access pointcomprising:

an array of antenna elements;

a digital beamforming weights matrix for applying a weightset one ormore signals streams;

a respective transmit chain for each antenna element; and

a processor configured to control the Equivalent Isotropic RadiatedPower (EIRP) emitted from antenna array in one or more beams in a firstmode of operation by:

calibrating the respective transmit chain for each antenna element interms of gain and phase;

providing a polar radiation model for an antenna element of the array;and

determining a weigthtset for the beamforming weights matrix subject to aconstraint that a maximum total EIRP for the one or more first beams incombination in any azimuth direction is maintained within apredetermined EIRP limit using the weightset, said determining beingbased at least on a spatial separation of the antenna elements, thepolar radiation model and the calibrated gain and phase of eachrespective transmit chain.

In an embodiment of the invention, the access point comprises:

a combiner network arranged to combine signals coupled from the outputof each transmit chain into a combined channel.

In an embodiment of the invention, the access point comprises an OFDMreceiver configured to receive an OFDM test symbol in the combinedchannel.

In an embodiment of the invention, the antenna array, the respectivetransmit chains and the combiner network are parts of a unit within asingle enclosure arranged to impede changing of the spacing of theantenna elements by an operator, whereby to maintain a predeterminedconfiguration of the antenna array to enable accurate determination ofmaximum EIRP.

In an embodiment of the invention, each respective transmit chain isconnected to the antenna array using printed conductors and without theuse of coaxial cable connectors.

In an embodiment of the invention, the circuit design and physicallayout of the radio frequency transmission paths are the same for eachantenna element.

This allows an accurate calculation of EIRP, because unknown radiofrequency characteristics will be the same for each antenna element andso may not affect a calculated array gain.

In an embodiment of the invention, the physical layout provides a fixedspacing between the radio frequency transmission paths for each antennaelement.

This allows an accurate calculation of EIRP.

Further features of the invention will be apparent from the followingdescription of preferred embodiments of the invention, which are givenby way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an EIRP limit and an access pointforming a sector beam or a beam to a subscriber unit within the EIRPlimit in an embodiment of the invention;

FIG. 2 is a schematic diagram showing an EIRP limit and an access pointforming three MU-MIMO beams, the combined EIRP of the MU-MIMO beamsbeing within the EIRP limit in an embodiment of the invention;

FIG. 3 is a schematic diagram showing a transmission architecture for anaccess point having a beamfoming weights matrix for a single datastream;

FIG. 4 is a schematic diagram showing a transmit chain;

FIG. 5 is a schematic diagram showing a transmission architecture for anaccess point having a beamfoming weights matrix for multiple datastreams;

FIG. 6 is a schematic diagram showing an access point having abeamfoming weights matrix, a calibrated module comprising transmitchains and antenna elements, and a processor for calculating a weightsetsubject to an EIRP constraint in an embodiment of the invention;

FIG. 7 is a schematic diagram showing a calibrated module comprisingtransmit chains and antenna elements in an embodiment of the invention;

FIG. 8 is a schematic diagram showing an architecture for transmissionof sounding tones into a module comprising transmit chains and antennaelements and receiving combined sounding tones from a receive chain inan embodiment of the invention;

FIG. 9 is a schematic diagram showing a receive chain;

FIG. 10 is a schematic diagram showing generation and combination ofsounding tones in the frequency domain in an embodiment of theinvention; and

FIG. 11 is a flow chart of a method of controlling the EIRP emitted froman array of antenna elements in an embodiment of the invention.

DETAILED DESCRIPTION

By way of example, embodiments of the invention will now be described inthe context of a fixed wireless access system operating a time divisionduplex system based on IEEE 802.11 standards at carrier frequenciestypically between 5 and 6 GHz. However, it will be understood that thisis by way of example only and that other embodiments may involve otherwireless systems and frequencies, and embodiments are not restricted toa specific frequency band of operation or a specific standard, and mayinvolve operation in licensed or unlicensed bands.

FIG. 1 is a schematic diagram showing an access point 1 according to anembodiment of the invention for use in a point to multipoint wirelesscommunication network comprising the access point and a plurality ofsubscriber modules 4 a, 4 b, 4 c. The access point operates within anEquivalent Isotropic Radiated Power (EIRP) limit 2, which appliesirrespective of the width of a radiated beam. The access point 1 has anarray of antenna elements, each element being arranged to transmit asignal that is appropriately weighted in amplitude and phase to form asector beam 5 within the EIRP limit in a first mode of operation, and toform a narrower directional beam 3 to a subscriber unit 4 b in a secondmode of operation, also within the limit. In FIG. 1, the radiated poweris indicated by the radius of the beam shape. The EIRP may be expressedas a power, which, if radiated by an ideal isotopic antenna, wouldproduce the same power per unit area at a given distance. For example,an EIRP limit may be +36 dBm, or 4 W.

FIG. 2 shows the access point 1 forming three MU-MIMO (Multiple UserMultiple Input Multiple Output) beams 6, 7, 8 in a third mode, thecombined EIRP of the MU-MIMO beams 9 being within the EIRP limit 2. In aMU-MIMO mode, a respective beam is formed for each of multiple datastreams using a beamforming weights matrix. So, for example, differentpayload data may be transmitted simultaneously to subscriber modules 4a, 4 b, 4 c. Each beam is typically arranged to form a null in thedirection of the other beams, so reducing interference between beams.The appropriately weighted signals for each beam are combined bysuperposition in the beamforming matrix to form a combined weightedsignal for transmission by each antenna element. Signals may beseparately combined to form a beam at each polarisation.

FIG. 3 is a schematic diagram showing a transmission architecture forthe access point. In this example, an input of one data stream is shown,data stream #1, for example for use in the sector mode, or thedirectional beamforming mode. The data stream is mapped to a series ofOrthogonal Frequency Division Multiplexing (OFDM) tones by mappingfunctional block 10. Two streams of OFDM tone values are created, A andB. If a polarisation diversity transmit scheme is used, then A will be aduplicate of B, so that the same data may be transmitted on bothpolarisations. This gives robust operation. If a polarising multiplexingapproach is taken, then the data stream will be split between steam Aand stream B of OFDM tone values, so that stream A and stream B aredifferent and each carries part of the data stream. This increases datacapacity. An OFDM tone value is a representation of an amplitude andphase of a tone, also referred to as a subcarrier, for an OFDM symbol.This may be typically a vector having an Inphase and Quadrature value.Data may be encoded in the tone value using QAM (Quadrature AmplitudeModulation) modulation.

Streams of OFDM tone values A and B are fed to the beamforming weightsmatrix 12. The beamforming weights matrix applies a weightset generatedby a beamforming function 11, for example using feedback from asubscriber module, to the streams of OFDM tone values. Typically eachOFDM tone value is weighted in amplitude and phase by a respectiveweight value for each nominally vertically polarised (V) andhorizontally polarised (H) component for each antenna element. Theweighting is typically performed using complex I (Inphase) and Q(Quadrature) components. The weight applied may be different fordifferent subcarriers, i.e. the weight may be frequency dependent. Theweightset is calculated by the beamforming function to form anappropriate beam shape when transmitted from the antenna array for theintended mode of operation; a broader beam for a sector mode and anarrower beam, directed at a subscriber module, for a directionalbeamforming mode. The weighted OFDM tone values for respective V and Hcomponents for respective antenna elements are fed to respectivetransmit chains 13 a-13 n.

FIG. 4 is a block diagram showing the components of a typical transmitchain. OFDM tone values, having been appropriately weighted, are appliedto IFFT block 15, which forms an OFDM symbol by applying an InverseFourier Transform to a set of OFDM tone values for subcarriers of thesymbol. Inphase and quadrature streams of time domain samples are formedat the output of the IFFT block. It is noted that the input to the IFFTblock is typically also in Inphase (I) and Quadrature (Q) form, but thisis shown in FIG. 4 by a single arrow to be compatible with the format ofFIG. 3, which also uses a single arrow to represent Inphase andQuadrature components. A cyclic prefix is added 16 a, 16 b to each ofthe Inphase and Quadrature streams of time domain samples for the symbolat the output of the IFFT block. Each stream of time samples isconverted to the analogue domain by a DAC (Digital to Analogue converter17 a, 17 b. Each analogue signal is then upconverted by up converter 18,which typically includes a complex IQ modulator and radio frequencymixers and amplifiers to translate the typically baseband Inphase andQuadrature signals up in frequency, using a radio frequency signalsource 20. The modulated signals are then typically amplified by poweramplifier 19, and fed to an appropriate polarisation input of an antennaarray element, typically a patch antenna, or a single antenna elementformed from an array of patches, for example a vertical array ofpatches.

Returning to FIG. 3, analogue signals at radio frequency are output fromrespective transmit chains and connected to respective antenna elementsof an array of antenna elements is shown 14 a-14 g, in this case anarray of 7 elements, each element having an input for transmission oneach of two orthogonal polarisations, in this case nominally vertical Vand horizontal H. Radiation from each antenna element combines to forman appropriately shaped beam. The weightsets applied for eachpolarisation may be independent, so that beams may be formedindependently on each polarisation.

FIG. 5 shows an access point architecture for multiple data streams. Inthis example, three data streams are shown: data stream #1, data stream#2 and data stream #3. The architecture is not limited to having threedata streams; only one data stream may be used for some modes, forexample for a sector mode or a combining mode. For MU-MIMO mode, theremay be any number of data streams up to the number of simultaneousMU-MIMO streams to be transmitted. A weightset for the beamformingweights matrix is applied to the data streams, by first mapping 10 a, 10b, 10 c each data stream to a stream of OFDM tone values, and thenapplying a respective weight from the weightset to each tone value foreach polarisation of each antenna element. Weighted tones for each datastream are combined together by superposition for transmission to arespective transmit chain.

Typically, it is difficult to calculate the maximum EIRP that wouldresult from applying a given weightset with a high degree of confidence,because there are many variable factors that affect the EIRP. It hasbeen found that the antenna radiation pattern of each antenna element,the isolation between antenna elements, the spatial arrangement of theantenna elements, and the gain and phase of the transmit chains andcable runs to the antenna elements affect the EIRP. The gain and phaseof the transmit chains may change with time, in particular as a functionof temperature, and the configuration of the antenna array and theconnection arrangement to the transmit chains is typically out of thecontrol of the manufacturer of the access point. An operator may replacecables, or use different antenna elements or a different spatialarrangement. As a result, in situations where a maximum EIRP limitapplies, it may be necessary to limit power output by design so that theEIRP will not be exceeded in the worst case. This has the disadvantagethat the access point will often be operating below the EIRP limit,which may limit data capacity of the radio system. For example, theconducted power to each antenna element may be limited to a value suchthat, if the maximum possible array gain were applied to an access pointconfigured to maximise the array gain, the EIRP limit would not beexceeded. This would, for example, reduce the EIRP of a beam that isconfigured with less than the maximum possible array gain.

FIG. 6 shows an arrangement in an embodiment of the invention at anaccess point for controlling the Equivalent Isotropic Radiated Power(EIRP) emitted from an array of antenna elements. The access point isconfigured to form one or more beams by applying a weightset for thebeamforming weights matrix 12 to one or more signal streams.

In a mode of operation, the method comprises calibrating thetransmission phase and gain of a transmit chain for each antennaelement. This may be achieved by the use of sounding tones as will bedescribed in connection with FIG. 10.

A weightset for the weighting matrix is determined to form the one ormore beams, but the determination is subject to a constraint that amaximum total EIRP for the one or more beams in combination in anyazimuth direction is maintained within a predetermined EIRP limit. Thedetermination is based at least on a known spatial separation of theantenna elements, a polar radiation model for an antenna element of thearray, for example a measured polar plot for one or more of the antennaelements, and the calibrated phase and gain of each respective transmitchain.

This allows the Equivalent Isotropic Radiated Power (EIRP) emitted froman array of antenna elements to be reliably controlled for a variety ofbeam shapes, so that the access point may transmit closer to an EIRPlimit without risking exceeding the limit. For example, the magnitude ofweight values may be operated at a greater level than would otherwise bethe case without control of EIRP to compensate for reduced array gain,for example in the case of a broader beam. Array gain is the signal gainresulting from coherent addition of transmitted signals radiated fromeach element of an antenna array. It has been found that, in particular,variations in the relative transmission phase of the transmit chains mayaffect the array gain. Calibration of the respective transmit chainsallows variations of phase and gain of each transmit chain, and so thegain and phase of the signal transmitted from each antenna element, tobe accurately accounted for when calculating array gain and so EIRP.Array gain in a given direction may be calculated by superposition ofthe relative gain and phase of signals transmitted from each antennaelement in that direction. This calculation takes into account thespatial arrangement of the antenna elements, and may also take intoaccount a polar radiation model of each antenna element. The polarradiation model of each antenna element may be the same, or a separatepolar radiation model may be used for each element. So, in particularthe magnitude of the radiation in the given direction may be taken intoaccount in calculating the array gain in that direction. EIRP may becalculated from array gain on the basis of a known absolute power levelof signals for transmission. Maximum EIRP for a beam may be determinedby calculating EIRP for a number of transmission directions, and findinga peak value of EIRP. Maximum EIRP for multiple beams, for exampleMU-MIMO beams, may be found by calculating the EIRP for each beam ateach of a number of directions, and combining the EIRP from the beams ateach direction to give a total EIRP for each direction, or for a sub-setof directions having highest EIRP. The maximum EIRP is then the Maximumcombined EIRP. If the maximum EIRP, which for, multiple beams, is themaximum combined EIRP, is above the EIRP limit, the weightset formingthe beam, or beams may be multiplied by a gain reducing factor, to bringthe maximum EIRP down to the limit, or to below the limit by a safetymargin factor. The entire weightset for combined beams may be multipliedby the gain reducing factor in a simple system, or only the weights forforming the beam which is causing the EIRP to exceed the limit may bereduced. Alternatively or in addition, the magnitude of the digitalsignal stream which is to be weighted may be reduced by the gainreducing factor. Constraining the maximum EIRP in this way allowsoperation closer to an EIRP limit than would be the case if the transmitchains were not calibrated, because in that case the array gain wouldnot be accurately known because the amplitude and phase of the signaltransmitted from each antenna array element would not be accuratelyknown. Optionally, if the maximum total EIRP is below the EIRP limit,then a gain increasing factor may be applied instead of the gainreducing factor to bring the EIRP closer to the limit.

As shown in FIG. 6, the first weightset may be determined by abeamforming function 11, that may optimise a signal to noise ratio to agiven subscriber module according to conventional beamformingtechniques. The first weightset may then be amended by a weightsetprocessing function 21, for example by applying a gain factor to theweightset, to maintain the EIRP within the predetermined limit whentransmitted. This allows the flexibility of generating narrow beamswhile maintaining operation within an EIRP limit. The beamformingfunction 11 and the weightset processing function 21 may be performed bya processor using software or hardware or firmware or a combination ofthese. Determining the weightset may be performed by determining a firstweightset for forming the one or more beams and amending the firstweightset so that the maximum total EIRP for the one or more beams incombination in any azimuth direction is maintained within thepredetermined EIRP limit.

The first weightset may be amended by determining the maximum total EIRPfor the one or more beams in combination in any azimuth direction, andcomparing the determined maximum total EIRP for the one or more beamswith a predetermined EIRP limit. If the maximum total EIRP of the one ormore beams is found to exceed the predetermined EIRP limit, the firstweightset is amended to reduce a gain factor for at least one of the oneor more signal streams, so that the maximum EIRP of the first beam isreduced to be within the EIRP limit. So, for example, if one MU-MIMObeam is found to exceed the EIRP limit, then the magnitude of the weightvalues in the weightset that apply to that beam may be reduced.

The calibrating of the respective transmit chains is as a function offrequency, and wherein the gain factor is a matrix of gain as a functionof frequency. This allows gain to be maintained at frequencies that donot exceed the EIRP limit, improving overall signal to noise ratio.

The beamforming weights matrix 12 of FIG. 6 operates in a similar mannerto that of FIGS. 3 and 5.

A shown in FIG. 6, the transmit chains and antenna elements are providedas a calibrated module 25.

FIG. 7 shows the calibrated module 25 which comprises transmit chainsand antenna elements in an embodiment of the invention. A coupler isprovided at the output of each transmit chain 13 a-13 n, which isarranged to couple a proportion of the signal power output from thetransmit chain from the connection from the transmit chain to theantenna element. The coupled signal power is connected to a signalcombiner 26, which combines the signals coupled from each transmit chaininto a single channel. The combined signal is fed to a receive chain 27.

The calibrated module 25 provides the antenna array, the respectivetransmit chains and the combiner network for the combined channel as anintegrated unit, typically in a single enclosure, arranged to impedechanging of the relative special arrangement of the antenna elements byan operator. The antenna elements of the antenna array may be formedfrom patch radiator elements, which are typically planar metallicstructures disposed in a parallel relationship to a ground plane. Thepatch radiator elements may be held in position in relation to theground plane by, for example, being printed on a non-conductive filmsuch as polyester, which is held in position on projections from theground plane. The ground planes of the antenna elements may be fixedtogether, or formed as a single piece, so that the relative spatialarrangement of the antenna elements cannot be changed in normal use byan operator. The radio frequency interconnections between the transmitchains 13 a-13 n and respective antenna elements 14 a-14 g may be formedof printed tracks on a printed circuit board, so that that signalpropagation properties will be stable with time. Similarly, therespective signal paths from the output of each transmit chain to thecombiner may be formed of printed tracks on a printed circuit board, andare typically passive. This allows a calibration, for example a factorycalibration, to be carried out for the gain and phase of each of thesignal paths from the output of each transmit chain to the output of thecombiner, and of the paths from each transmit chain to the respectiveantenna elements. This calibration may be used in the calibration of thetransmission phase and gain of each transmit chain.

Connecting each respective transmit chain to the antenna array withoutuse of coaxial cable connectors allows accurate determination of EIRP tobe maintained after delivery to an operator by preventingreconfiguration of the equipment.

The calibrated module 25 may typically use a symmetrical design. Thebenefit of a symmetrical design is that unknown but symmetricalcharacteristics such as PA droop and phase twist, feed delays and so onare not important in calculating EIRP if they are the same for allchains. This translates into a symmetrical hardware design where allelements of the analogue signal paths, including feed networks, RFchains, and PCB layout are identical and repeated with a fixed spacing.

FIG. 8 is a schematic diagram showing an architecture for transmissionof sounding tones for use in calibrating the calibrated module 25comprising transmit chains and antenna elements and for receivingcombined sounding tones from a receive chain in an embodiment of theinvention. Sounding tones, typically predetermined OFDM subcarrieramplitude and phase values to be used for test purposes, includingcalibration of the transmit chains, are transmitted from a sounding tonetransmitting functional block 28. A processor 29 may hold the soundingtone values in memory. The sounding tones may be sent by a processorthrough the beamforming weights module, with weights set topredetermined values, for connection to the transmit chains, in whichthe frequency domain tones are converted to time domain soundingsymbols.

The sounding symbols coupled from the output of each transmit chain arecombined in combiner 26 and the combined sounding symbols are connectedto receive chain 27, and the output of the receive chain, comprisingamplitude and phase values for each received tone of the symbols, areconnected to processor 29. The processor compares the amplitude andphase of the transmitted and received tones, generating calibration datafor the respective transmit chains, taking into account the calibratedradio frequency paths from the output of each transmit chain through thecombiner and the receive chain.

FIG. 9 is a block diagram showing components of a typical receive chain27. The combined sounding symbols are amplified by low noise amplifier32, and then down converted from radio frequency, typically 5-6 GHz, bydownconverter 33, using radio frequency source 35. The signals are downconverted typically to baseband in Inphase and Quadrature components.The baseband signals are then converted to the digital domain in theAnalogue to Digital Converter (ADC) 34 a, 34 b. The cyclic prefix, ifused, is discarded 36 a, 36 b and the received symbol is then translatedto the frequency domain using Fast Fourier Transform (FFT) 37, to detectamplitude and phase values of each sounding tone. The processor 29 maycompare amplitude and phase values of the received tones with thetransmitted amplitude and phase values of each tone, to calibrate thetransmission phase and gain of each transmit chain. Relative transmitphases and gains of the transmit chains may be calculated.

FIG. 10 shows an example of generation and combination of sounding tonesin the frequency domain in an embodiment of the invention. In thisexample, an OFDM test symbol is generated for each transmit chain, eachrespective OFDM test symbol comprising a respective set of energisedsubcarriers, that is to say sounding tones. In FIG. 10, typical soundingtones are shown for transmit chain 1V and 1H 30 a, 30 b. The soundingtones for a given transmit chain are not used for the other transmitchains. This allows the test symbols to be received without interferencebetween the test symbols. The signals from each transmit chain arecombined into a combined channel, and a combined OFDM symbol is receivedin the combined channel, the combined OFDM symbol comprising respectivesubcarriers, that is to say sounding tones 31, transmitted by respectivetransmit chains. Each transmit chain is calibrated based on the receivedrespective subcarriers in the combined OFDM symbol. This allows a simplereceiver architecture to be implemented using a combiner, and reducestest time by enabling the test symbols to be received simultaneouslywithout interference between the test symbols.

As an alternative to the OFDM test symbols illustrated by FIG. 10, othersounding tones may be used, provided that it is possible to determinethe transmission amplitude and phase for each transmit chain from thecombined symbol or a series of combined symbols. For example, thesounding tones may be arranged such that the relationship between OFDMtest symbols may be characterised by a Hermitian matrix, so thatorthogonal results may be derived for each channel.

The calibrating of the transmission phase and gain of respectivetransmit chains may be performed periodically as part of a time framesequence including time frames for the transmission of payload data.This allows variations of the gain and/or phase of the transmit chainswith time and/or temperature to be calibrated.

The period between performance of calibration may be less than or equalto 64 time division duplex frames, and may be 8 to 32 time divisionduplex frames, typically 16 frames. This has been found to offer a goodtrade-off between calibration accuracy and throughput of payload data,which may be inhibited during calibration.

In a time division duplex system, downlink signals transmitted from anaccess point and uplink signals transmitted from a subscriber module aretransmitted at the same frequency. Alternating fixed-duration timeperiods, known as time division duplex frames, are allocated for uplinkand downlink transmission respectively. A time division duplex frame istypically divided into timeslots, each timeslot typically being forcommunication with a subscriber module, or in the case of MU-MIMOoperation, with a group of subscriber modules. The access point mayswitch from one mode of operation to another between timeslots, forexample from sector mode to MU-MIMO mode. Calibration of transmit chainsusing of sounding tones may be performed within a timeslot.

The access point may switch from a first mode of operation to a secondmode of operation, potentially in consecutive timeslots. In the secondmode of operation, one or more beams may be configured by applying asecond weightset for the beamforming weights matrix to one or moresignal streams. The second weightset may be determined subject to aconstraint that a maximum total EIRP for the one or more beams incombination in any azimuth direction is maintained within the samepredetermined EIRP limit as for the first mode, allowing flexibility ofoperation in terms of beamwidth while maintaining transmitted powerwithin an EIRP limit. For example, the first mode may be a sector modein which signals from the respective transceiver chains are combined toform a beam sufficiently broad to provide coverage of a sector of acellular radio network, the second mode is a combining mode, in whichsignals from the respective transceiver chains are combined to form abeam which is narrower in azimuth than that formed in the sector mode toprovide a beam steered to an individual subscriber mode within thesector of a cellular radio network. This allows the flexibility ofswitching to a beamformed mode of operation, for example forcommunication to a given subscriber module, while maintainingtransmitted power within an EIRP limit.

Alternatively, the first mode may be the sector mode the second mode maybe a Multiple User Multiple Input Multiple Output (MU-MIMO) mode, inwhich signals from the respective transceiver chains are combined toform at least two beams carrying different data to respective subscribermodules within the sector of a cellular radio network. This allows theflexibility of switching to a MU-MIMO mode of operation, for example forcommunication of respective data streams to several subscriber modules,while maintaining total transmitted power for the MU-MIMO beams withinan EIRP limit in all azimuth directions.

FIG. 11 is a flow chart of a method of controlling the EIRP emitted froman array of antenna elements in an embodiment of the invention,comprising steps S11.1 and S11.2. It will be understood that the methodof embodiments of the invention may be implemented by a processor, whichmay comprise program code held in a memory configured to cause theprocessor to perform the method. The processor may comprises one or moredigital signal processors, and/or programmable logic arrays. Theprocessor may be configured to control the Equivalent Isotropic RadiatedPower (EIRP) emitted from antenna array in one or more beams in a firstmode of operation by calibrating the respective transmit chain for eachantenna element in terms of gain and phase, providing a polar radiationmodel for an antenna element of the array, and determining a weightsetfor the beamforming weights matrix subject to a constraint that amaximum total EIRP for the one or more first beams in combination in anyazimuth direction is maintained within a predetermined EIRP limit usingthe weightset, said determining being based at least on a spatialseparation of the antenna elements, the polar radiation model and thecalibrated gain and phase of each respective transmit chain.

Each weightset may comprise respective amplitude and phase values forrespective signal streams for respective antenna elements for respectivesub-carriers of an OFDM symbol. This allows beamforming to take intofrequency dependent effects.

The isolation between antenna elements may be at least 30 dB. Thisallows an accurate model of EIRP to be determined without modellinginteraction between antenna elements, so that the access point may beoperated nearer to a predetermined EIRP limit.

In an embodiment of the invention, the circuit design and physicallayout of the radio frequency transmission paths are the same for eachantenna element. This allows an accurate calculation of EIRP, becauseunknown radio frequency characteristics will be the same for eachantenna element and so may not affect a calculated array gain. In anembodiment of the invention, the physical layout of said unit provides afixed spacing between the radio frequency transmission paths for eachantenna element. This allows an accurate calculation of EIRP.

In a fixed wireless access system the subscriber module may be typicallymounted to a structure such as a building, typically on the outside of abuilding in a position that gives good radio reception to an accesspoint. The access point 1 may be located at a convenient point to servea number of subscriber units. For example the access point, or theantennas for the access point, may be mounted on an antenna tower, andmay provide Internet access to a neighbourhood.

The subscriber modules 4 a, 4 b, 4 c shown in FIG. 1 may have antennaswhich have an aperture defined for example by a reflector, and eachantenna element may comprise a probe for receiving and/or transmitting arespective polarisation from/to the aperture. The antenna is typicallyinstalled so as to align the peak of the transmit/receive radiationpattern in the direction of the access point 1, which is typicallyinstalled on a tower. A command sent to each subscriber module maycomprise a map indicating a scheduling of radio resource and/orpolarisation to the subscriber module as a function of time. The map mayindicate respective allocations to several subscriber units as afunction of time, typically all subscriber units served by an accesspoint. The map may indicate, for example, time, polarisation, and/orfrequency allocation for transmission and/or reception. The schedulingof radio resource and polarisation may be updated periodically, theperiod between updates being determined by a scheduler at the accesspoint.

A specific example of an access point according to an embodiment of theinvention is given by a point-to-multipoint (PMP) Access Point (AP) witha seven-element dual-polarised adaptive array smart antenna andmulti-user MIMO (MU-MIMO) capabilities. This example will now bedescribed in more detail. It will be understood that embodiments of theinvention are not limited to this example. The access point in thisexample is designed for outdoor deployment as an AP with sector coveragein a PMP network. Units may be deployed in multiples to provide 360°coverage from a tower or rooftop. The access point may be a completeradio transceiver operating in the frequency range 5150 MHz to 5925 MHz,using Time Division Duplex (TDD) separation of the uplink and downlinkdirections.

The access point may include an integrated dual-polarised seven-elementadaptive array smart antenna. Seven identical dual-polarised antennaelements and 14 associated transceiver chains may be contained within asingle rigid assembly, with each antenna element connected directly totwo transceiver chains using printed conductors and wireless viaconnections. The integration of the components ensures that the spacingand alignment of the antenna elements is known and constant.

Each antenna element may consist of a vertical column of eight radiatingpatches and separate passive feed networks for horizontal and verticalpolarisations. A single element may have a relatively narrow beamwidth(about 8°) in the elevation direction, and a broader beamwidth (about80°) in the azimuth direction. The gain of each antenna element (that isto say, each column of eight patches) is about 14 dBi. The overallantenna assembly may contain 56 patches, in an array that is sevenelements (seven patches) wide and one element (eight patches) high.

The antenna array may provide high isolation between antenna elements.The coupling loss between antenna elements may be greater than 30 dB;this enables the device to model smart antenna operation moreaccurately.

The integrated assembly in this example does not make use of anyconnectors between the antenna elements and the associated electronics,and does not provide any test points that could be used to makeconducted measurements.

In this example, the maximum output power of a single transmitter chainis about 10 dBm, or 13 dBm for each dual-polarised pair of chains

The associated Subscriber Module (SM) devices may contain a directionaldual-polarised antenna with two transceiver chains. The SMs may supporta single data stream using polarisation diversity or polarisationmultiplexing. In MU-MIMO operation, the AP may support several (up toseven in this example) parallel data streams, where each stream isassociated with a different SM device.

The access point in this example supports four distinct smart antennamodes: Combining mode; Sector mode; MU-MIMO mode; and Sounding mode.

The access point in this example uses the combining smart antenna modewhen communicating with a single SM over a known channel. Thebeamforming action produces a pattern that is significantly narrower inthe azimuth direction than that of a standard sector antenna, allowingthe main antenna response to be steered to an individual SM by varyingthe digital amplitude and phase weights in the 14 chains. The narrowerbeamwidth in this mode helps to reduce the inter-cell uplinkinterference level received at the AP compared with reception using astandard sector antenna. When deployed throughout a network, thenarrower downlink beamwidth also tends to reduce overall inter-cellinterference levels at SMs.

Control functions in the AP may automatically adjust digital transmitgain to compensate for array gain in the combining mode, ensuring thatthe radiated power is never greater than the power allowed by therelevant rules. The gain adjustment may be determined using an accuratemodel of smart antenna operation based on a frequency-dependent model ofthe polar response of a single antenna element. The model increasesdigital gain as the beam is steered away from the centre in order tomaintain the configured EIRP across a range of azimuth angles.

The reduction in the drive level that arises as a consequence ofutilising array gain has the beneficial outcome of reducing transmitterdistortion and thereby contributing to the use of the most efficientmodulation modes in the downlink direction.

The access point may use the sector mode when transmitting broadcastdata, or when receiving from an SM over an unknown channel. Theamplitude and phase weights of the individual chains are selected toprovide sector coverage, meaning that overall array gain is close tounity. The advantage of the sector mode compared to using a singleantenna element, is that it allows the device to exploit the combinedtransmitter power of all 14 chains.

The access point may use the MU-MIMO mode to transmit and receive datain several parallel streams where each stream involves a different SM.

The MU-MIMO operation consists of beamforming to maximise the uplink anddownlink signal in one stream for each wanted SM, and null-steering tominimise the uplink and downlink signals for SMs that are associatedwith the other parallel streams. The resulting antenna beams willnecessarily be at different azimuth angles such that the antenna beamsare substantially non-overlapping.

The MU-MIMO smart antenna mode may be invoked when suitable orthogonalgroups of SMs have been identified, and where buffered data is queued,ready to be transmitted to or received from these SMs.

Control functions in the AP may automatically reduce the digitaltransmit gain to compensate for array gain in the MU-MIMO mode, ensuringthat the radiated power at any azimuth angle is less than the powerallowed by the relevant rules.

The access point may use the Sounding smart antenna mode to characterisethe channel between each of the antenna elements and each of the SMs.The Sounding mode is also used to calibrate the gain and phase of eachof the AP transmit chains.

In the Sounding smart antenna mode each OFDM tone may be energised inonly one of the 14 chains in this example. It follows from this that allthe smart antenna outputs are intrinsically uncorrelated in this smartantenna mode.

For the remaining smart antenna modes, the access point may support twoMIMO modes, namely: Polarisation diversity, using cyclic delay diversity(CDD); and Polarisation multiplexing.

In the polarisation diversity MIMO mode, the same data is present inboth polarisations during the same symbol period, and the two channelsare therefore considered to be partially correlated. In the polarisationmultiplexing MIMO mode, the data stream is shared between the twopolarisations, and the two channels are therefore considered to becompletely uncorrelated.

The selection of the MIMO mode is, in principle, independent of theselection of the smart antenna mode. However, the channel conditionsneeded for MU-MIMO operation are similar to the channel conditionsneeded for polarisation multiplexing, and the combination ofpolarisation diversity and MU-MIMO operation may occur relativelyrarely.

Returning to FIG. 3, this shows a block diagram for single stream(sector or combining) operation of the transmit direction in the smartantenna. A single input serial data stream at the left of the figure hasbeen encrypted, encoded for forward error correction (FEC) and scrambledto ensure a known density of transitions in earlier stages not shown inthe figure. The first stage shown here maps a sequence of the serialdata into separate A and B channels using polarisation diversity (wherethe same data is present in each channel) or polarisation multiplexing(where the data is divided between two channels), and for each the twochannels maps the data into the amplitude and phase coordinates of a setof OFDM tones representing a single OFDM symbol.

The coordinates of the A and B tone sets may then be each multiplied bya set of 14 amplitude and phase weights generated by a beamformingfunction to create seven H and seven V inputs to the following stage.

In each of the 14 transmitter chains, the weighted tone sets are passedto the IFFT stage to generate a series of in-phase and quadraturetime-domain samples for an OFDM symbol. The unit then adds a complexcyclic prefix to the time domain signals and converts the I and Qsignals to analogue waveforms. The analogue signals are applied to anup-converter to provide the modulated RF output.

The 14 modulated RF signals are then amplified and applied in pairs tothe H and V ports of the seven dual-polarised antenna elements.

Returning to FIG. 5, this shows the block diagram for the weights matrixin the transmit direction for MU-MIMO operation. The weights matrix issimilar to the matrix in FIG. 3 except that the unit supports multipleindependent data streams, and each data stream is applied to the 14transceiver chains according to the amplitude and phase weights.

FIG. 5 shows three data streams, so that the weights matrix hasdimension 6×14. The unit supports up to seven parallel data streams,meaning that the weights matrix could have dimension 14×14.

The Sounding mode may be used to calibrate the gain and phase of each ofthe AP transmit chains up to the output of the RF power amplifiers. Theamplitude of the transmitted signal is determined by coupling all of thetransmitter signals into an accurate detector, and by passing thecomposite signal into an additional OFDM receiver stage. The device isable to calibrate each transmitter chain by considering the amplitudeand phase of each of the 14 sets of OFDM tones separately.

Transmitter gain may be adjusted by a combination of analogue gainadjustments in the RF stages and digital gain and phase adjustments inthe calculation of weights in the combining matrix. Analogue gain may beadjusted in a calibration sequence at initialisation of the device, andwhenever the maximum transmitted power is changed by the operator.Thereafter, adjustments may be made solely by changing the digital gainin the weights matrix, except that an additional analogue adjustment mayoccasionally be needed to maintain the desired dynamic range of thedigital signals. Analogue adjustments may be avoided as far as possiblebecause MU-MIMO operation must be temporarily suspended whilst theadjustment is made.

The application firmware in the access point may compute the transmittergain, and amplitude and phase weights for the combining matrix, toprovide the required sector, single beam or MU-MIMO beam patterns. Thisoperation is based on an accurate model of smart antenna operation, inwhich the resultant signal strength at any azimuth angle is determinedas the superposition of the signals radiated by the individual antennaelements. This model may automatically and intrinsically allow for arraygain in the smart antenna.

The accuracy of the prediction of the behaviour of the system by themodel for smart antenna operation may be contributed to by the followingfactors: the seven antenna elements and the 14 transmitter chains aresubstantially identical; the spacing between the antenna elements isequal and fixed; the antennas cannot be changed by an installer, andthere are no antenna cables that could be changed or disconnected; themodel includes the frequency-dependent polar response of a singleantenna element; the amplitude and phase response of the transmitterchains is regularly calibrated; the calibration process measures andcounters differences between chains that arise because of manufacturingspread, frequency or operating temperature; and the antenna elements areeffectively isolated from each other so that interaction betweenelements is minimal, so that the assumption of superposition isrealistic.

The firmware may compute the analogue gain and digital combiner weightssubject to the constraint that the resultant radiated power at the peakof the beam (including the effect of array gain) must not exceed themaximum radiated power configured by the operator. The maximum radiatedpower that an operator can configure is capped at the maximum allowed bythe applicable rules, ensuring that the unit will comply with rules forradiated power and radiated power density at any azimuth angle and forany combination of SM locations.

The above embodiments are to be understood as illustrative examples ofthe invention. It is to be understood that any feature described inrelation to any one embodiment may be used alone, or in combination withother features described, and may also be used in combination with oneor more features of any other of the embodiments, or any combination ofany other of the embodiments. Furthermore, equivalents and modificationsnot described above may also be employed without departing from thescope of the invention, which is defined in the accompanying claims.

1. A method of controlling the Equivalent Isotropic Radiated Power(EIRP) emitted from an array of antenna elements at an access point of awireless communication network, the access point being configured toform one or more beams by applying a respective weightset for a firstand second mode of operation for a beamforming weights matrix to one ormore signal streams, the method comprising: calibrating transmissionphase and gain of a respective transmit chain for each antenna element;providing a polar radiation model for an antenna element of the array;determining a first weightset for the weighting matrix for a first modeof operation subject to a constraint that a maximum total EIRP for theone or more beams in combination in any azimuth direction is maintainedwithin a predetermined EIRP limit; and switching from the first mode ofoperation to the second mode of operation, wherein the first mode is asector mode in which signals from the respective transceiver chains arecombined to form a beam sufficiently broad to provide coverage of asector of a cellular radio network, and the second mode is a mode inwhich signals from the respective transceiver chains are combined toform at least one beam which is narrower in azimuth than that formed inthe sector mode, the at least one beam being steered to an individualsubscriber mode within the sector of a cellular radio network; and inthe second mode of operation, configuring one or more beams by applyinga second weightset for the beamforming weights matrix to the one or moresignal streams, wherein the second weightset is determined subject to aconstraint that a maximum total EIRP for the one or more beams incombination in any azimuth direction is maintained within the samepredetermined EIRP limit as for the first mode, wherein saiddetermining, in each mode of operation, is based at least on a spatialseparation of the antenna elements, the polar radiation model and thecalibrated transmission phase and gain of each respective transmitchain.
 2. The method according to claim 1, wherein said determiningcomprises: determining a first weightset for forming the one or morebeams; and amending the first weightset so that the maximum total EIRPfor the one or more beams in combination in any azimuth direction ismaintained within the predetermined EIRP limit.
 3. The method accordingto claim 2, wherein amending the first weightset comprises: determiningthe maximum total EIRP for the one or more beams in combination in anyazimuth direction; comparing the determined maximum total EIRP for theone or more beams with a predetermined EIRP limit; in dependence on themaximum total EIRP of the one or more beams exceeding the predeterminedEIRP limit, amending the first weightset to reduce a gain factor for atleast one of the one or more signal streams to reduce the maximum EIRPof the first beam to be within the EIRP limit.
 4. The method accordingto claim 1, wherein said calibrating of the respective transmit chainsis as a function of frequency, and wherein the gain factor is a matrixof gain as a function of frequency.
 5. The method according to claim 4,wherein said calibrating of the respective transmit chains comprises:generating an Orthogonal Frequency Division Multiplexing (OFDM) testsymbol for each transmit chain: combining signals from each transmitchain into a combined channel; receiving a combined OFDM symbol in thecombined channel, the combined OFDM symbol comprising respectivesubcarriers transmitted by respective transmit chains; and calibratingeach transmit chain based on the received respective subcarriers in thecombined OFDM symbol.
 6. The method according to claim 5, wherein eachrespective OFDM test symbol comprises a respective set of energisedsubcarriers.
 7. The method according to claim 5, wherein a relationshipbetween the OFDM test symbols is characterised by a Hermitian matrix. 8.The method according to claim 1, comprising providing an isolationbetween antenna elements of at least 30 dB.
 9. The method according toclaim 1, comprising performing said calibrating of the transmissionphase and gain of respective transmit chains periodically as part of atime frame sequence including time frames for the transmission ofpayload data.
 10. The method according to claim 9, wherein the periodbetween performance of calibration is less than or equal to 64 timedivision duplex frames.
 11. The method according to claim 1, comprisingproviding the antenna array, the respective transmit chains and acombiner network for the combined channel as a unit within a singleenclosure arranged to impede changing of the relative specialarrangement of the antenna elements by an operator, whereby to maintaina predetermined configuration of the antenna array to enable accuratedetermination of maximum EIRP.
 12. (canceled)
 13. (Cancelled)
 14. Themethod according to claim 1, wherein: the second mode is a Multiple UserMultiple Input Multiple Output (MU-MIMO) mode, in which signals from therespective transceiver chains are combined to form at least two beamscarrying different data to respective subscriber modules within thesector of a cellular radio network, wherein an array gain is controlledby a gain factor so that a combined EIPR of the MU-MIMO beams in anyazimuth direction is maintained within the predetermined EIRP limit. 15.The method according to claim 1, wherein each weightset comprisesrespective amplitude and phase values for respective signal streams forrespective antenna elements for respective sub-carriers of an OFDMsymbol.
 16. An access point for a wireless communication network, theaccess point comprising: an array of antenna elements; a digitalbeamforming weights matrix for applying a weightset to one or moresignals streams; a respective transmit chain for each antenna element;and a processor configured to control an Equivalent Isotropic RadiatedPower (EIRP) emitted from the antenna array in one or more beams by:calibrating transmission phase and gain of a respective transmit chainfor each antenna element providing a polar radiation model for anantenna element of the array; determining a first weightset for theweighting matrix for a first mode of operation subject to a constraintthat a maximum total EIRP for the one or more beams in combination inany azimuth direction is maintained within a predetermined EIRP limitand switching from the first mode of operation to the second mode ofoperation, wherein the first mode is a sector mode in which signals fromthe respective transceiver chains are combined to form a beamsufficiently broad to provide coverage of a sector of a cellular radionetwork, and the second mode is a combining mode, in which signals fromthe respective transceiver chains are combined to form at least one beamwhich is narrower in azimuth than that formed in the sector mode, the atleast one beam being steered to an individual subscriber mode within thesector of a cellular radio network; in the second mode of operation,configuring one or more beams by applying a second weightset for thebeamforming weights matrix to the one or more signal streams, whereinthe second weightset is determined subject to a constraint that amaximum total EIRP for the one or more beams in combination in anyazimuth direction is maintained within the same predetermined EIRP limitas for the first mode, wherein said determining, in each mode ofoperation, is based at least on a spatial separation of the antennaelements, the polar radiation model and the calibrated transmissionphase and gain of each respective transmit chain.
 17. The access pointaccording to claim 16, comprising: a combiner network arranged tocombine signals coupled from the output of each transmit chain into acombined channel.
 18. The access point according to claim 17, whereinthe antenna array, the respective transmit chains and the combinernetwork are parts of an integrated unit arranged to impede changing ofthe spacing of the antenna elements by an operator, whereby to maintaina predetermined configuration of the antenna array to enable accuratedetermination of maximum EIRP.
 19. The access point according to claim18, wherein each respective transmit chain is connected to the antennaarray using printed conductors and without the use of coaxial cableconnectors.
 20. The access point according to claim 19, wherein thecircuit design and physical layout of the radio frequency transmissionpaths are the same for each antenna element, and wherein the physicallayout of said unit provides a fixed spacing between the radio frequencytransmission paths for each antenna element.